Added assignment operator/copy constructor for contexts (without this clang produces incorrect code)
Added assignment operator/copy constructor for contexts (without this clang produces incorrect code)

nathanielbaxter/libminizinc,nathanielbaxter/libminizinc,jjdekker/libminizinc,jjdekker/libminizinc,jjdekker/libminizinc,nathanielbaxter/libminizinc,tias/libminizinc,jjdekker/libminizinc,tias/libminizinc,nathanielbaxter/libminizinc